Nuclear fuel elements typically comprise radioactive fuel materials contained within a hermetic or gastight sealed cladding. The sealed cladding is utilized to contain the gaseous by-products of the fission process. However, as the reaction continues in a reactor, these gaseous fission by-products build up within the sealed cladding. The resulting pressurization due to retention of large amounts of gaseous products within a limited interior volume can lead to cladding rupture. This is particularly true in the case of cladding in the form of thin-walled tubing. The useful life of fuel elements does not necessary depend upon the time during which it can sustain a chain reaction, but upon the risk of potential cladding failure.
The reactivity lifetime of a nuclear fuel element is affected by the nature and quantity of cladding materials about the fuel, as well as by the progressively developed quantities of fission by-products. In designing a fuel assembly, one must balance the thickness of the cladding material necessary to insure against cladding rupture against the increasing parasitic absorption of neutrons, which is directly proportional to cladding thickness. While the use of thicker cladding might permit the containment or retention of gaseous by-products at higher pressures, this leads to increased parasitic absorption of neutrons by both the cladding and the gaseous by-products.
In newer forms of fast reactors, thin cladding is desired to reduce the parasitic absorption of neutrons and to minimize flux moderation. The thinner cladding in turn reduces the amount of gaseous pressure that can be permitted to build up within each fuel element. Under conventional practice, such design factors greatly shorten the reactivity lifetime of a fuel element between reprocessing cycles.
The present invention is designed to permit use of thinner cladding, reduced parasitic neutron absorption by both the cladding and gaseous fission by-products, and longer potential in-reactor service of fuel elements with reduced refabrication frequency. This is accomplished by periodic venting of each fuel element to release the gaseous fission by-products. This periodic release of the gaseous fission by-products within the cladding tubes further permits the design of reactors using shorter fuel assemblies and generally more compact operating and handling equipment in all parts of the reactor relating to the fuel elements.